We will analyze the actions of genes that regulate how photoreceptor synapses in Drosophila form and function, and how these contribute to circuits of neurons in the fly's visual system. Projects focus on the photoreceptor target interneuron in the first and second neuropils, the lamina and medulla, functional counterparts to the outer and inner plexiform layers of the retina. A long-term objective is to understand the organization of multiple-contact synapses (such as dyads and triads) from the fly's photoreceptor tetrad synapses. More recent objectives are to understand how these form and then contribute to the visual system's synaptic microcircuits. Current objectives are to study axon targeting prior to photoreceptor synaptogenesis, and the action of crumbs in directing growth cone trajectories. The numbers and types of feedback photoreceptor synapses will be studied using genetic reagents to label and identify participating neurons at EM level, and the involvement of Kirre and Irrec-like proteins as well as Dscam cell adhesion molecules in establishing the specificity of synapses between two types of lamina feedback interneuron, L2 and L4, as well as in regulating reciprocity between these. The projects use mutants and knockdowns of the corresponding genes, and other genetic reagents. Synaptic function will be examined from mutants that alter the synaptic vesicle phenotype of photoreceptors, or the targeting of their terminals, as well as the recycling of neurotransmitter, histamine, through a beta-alanyl conjugation pathway. Pathway strength for photoreceptor feedback will be evaluated from synapse numbers in different mutant backgrounds to reveal the network regulation of synaptic circuits. These projects examine mutant photoreceptors using advanced methods of serial-section and immuno-EM, and skilled personnel to implement these. Analysis of synaptic circuits in the complex medulla will continue, using serial-EM to identify actual circuits between identified neurons of the pathways underlying spectral discrimination. These data will be used to examine the neural basis of color vision; to identify circuit design and the frequencies of network motifs in synaptic circuits; and to examine the regulation that these undergo in both the lamina and medulla when contributing neurons are eliminated or genetically transformed. The proposed studies will identify the cellular mechanisms for synaptic function and organization in visual systems, and the rearrangements these undergo in functional and disease states of the retina, and will be identified from their genetic bases in a model visual system with marked similarities to the retina.